ROSR, A DETERMINANT OF NODULATION COMPETITIVENESS IN RHIZOBIUM-ETLI

We previously described a Tn5 mutant of Rhizobium etli strain CE3, des ignated CE3003, that is decreased in nodulation competitiveness, reduc ed in competitive growth in the rhizosphere, and has a hydrophobic cel l surface (R. S. Araujo, E. A. Robleto, and J. Handelsman, Appl. Envir on, Microbiol., 60:1430-1436, 1994), To determine the molecular basis for the mutant phenotypes, we identified a 1.2-kb fragment of DNA deri ved from the parent that restored the wild-type phenotypes to the muta nt. DNA sequence analysis indicated that this 1.2-kb fragment containe d a single open reading frame that we designated rosR. The Tn5 inserti on in CE3003 was within rosR. We constructed a derivative of CE3 that contained a deletion in rosR, and this mutant was phenotypically indis tinguishable from CE3003 in cell surface and competitive characteristi cs. Based on the nucleotide sequence, the deduced RosR amino acid sequ ence is 80% identical to that of the Ros protein from Agrobacterium tu mefaciens and the MucR protein from Rhizobium meliloti. Both Ros and M ucR are transcriptional repressors that contain a putative zinc-finger DNA-binding domain, This study defines a gene, rosR, that is homologo us to a family of transcriptional regulators and contributes to nodula tion competitiveness of R. etli. Moreover, we established that a singl e gene affects nodulation competitiveness, competitive growth in the r hizosphere, and cell surface hydrophobicity.